It is well known that silver has desirable electrical and thermal conductivity, catalytic properties, and antimicrobial behavior. Thus, silver and silver-containing compounds have been widely used in alloys, metal plating processes, electronic devices, imaging sciences, medicine, clothing or other fibrous materials, and other commercial and industrial articles and processes to take advantage of silver's beneficial properties.
For example, silver compounds or silver metal have been described for use as metallic patterns or electrodes in metal wiring patterns, printed circuit boards (PCBs), flexible printed circuit boards (FPCs), antennas for radio frequency identification (RFID) tags, plasma display panels (PDPs), liquid crystal displays (LCDs), organic light emitting diodes (OLEDs), flexible displays, and organic thin film transistors (OTFTs), among other electronic devices known in the art.
Rapid advances are also occurring for making and using various electronic devices for communication, financial, and archival purposes.
Silver is an ideal conductor having electrical conductivity 50 to 100 times greater than indium tin oxide that is commonly used today in many devices. For example, the art has described the preparation of electrically-conductive films by forming and developing (reducing) a silver halide image in “photographic” silver halide emulsions through an appropriate mask to form electrically-conductive grid networks having silver wires having average sizes (width and height) of less than 10 μm and having appropriate lengths.
While silver as an electrical conductor has a wide range of potential uses in the field of printed electronics, the microfabrication of electrically-conductive tracks (grids, wires, or patterns) by photolithographic and electroless techniques is time consuming and expensive, and there is an industrial need for direct digital printing to simplify the processes and to reduce manufacturing costs.
Furthermore, it is desirable to fabricate silver-containing electronics onto polymeric or similar temperature-sensitive substrates by solution-based printing processes. Metallic electrically-conductive wires or grids of low resistance must be achieved at sufficiently low temperatures to be compatible with organic electronics on polymeric substrates. Among various known methods for fabricating electrically-conductive silver grids or patterns, the direct printing of silver-containing inks provides attractive prospects for making such electrically-conductive patterns.
Inkjet printing and flexographic printing have also been proposed for providing patterns of silver or silver-containing compounds, requiring the careful fabrication of a silver-containing paste or “ink” with desirable surface tension, viscosity, stability, and other physical properties required for such application processes. High silver content has generally been required for high electrical conductivity, and calcination or sintering may be additionally required for increasing electrical conductivity of printed silver inks.
Some approaches to providing silver metal is to employ a chemical ink formulation where the silver source is a molecular precursor or cation (such as a silver salt) that is then chemically reacted (or reduced) to produce silver metal. Electrically-conductive inks that are in the form of a chemical solution rather than as a suspension or dispersion of metal particles, have gained interest in recent years. One conductive ink of this type is known as a Metalorganic Decomposition (MOD) variety ink, for example, as described by Jahn et al. [Chem. Mater. 22, 3067-3071 (2010)] who investigated silver printing using an aqueous transition metal complex [AgO2C(CH2OCH2)3H]-containing MOD ink. They reported the formation of metallic silver features having electrical conductivities as high as 2.7×107 S m−1, which corresponds to an electrical conductivity that is 43% of that of bulk silver, although a sintering temperature of 250° C. was required.
U.S. Patent Application Publication 2015-0004325 (Walker et al.) describes a chemically-reactive silver ink composition comprised of a complex of a silver carboxylate salt and an alkylamine, in which the complex is used to form an electrically-conductive silver structure at a temperature of 120° C. or less. Unfortunately, even these temperatures render the ink incompatible with many polymeric and paper substrates used in flexible electronic and biomedical devices. Furthermore, since alkylamines are known to reduce silver at room temperature, long term stability of such compositions is tentative. Furthermore, the publication teaches long heating times were needed to obtain low resistivity in the resulting articles.
Other industrial approaches to preparing electrically-conductive films or elements have been directed to formulating and applying photocurable compositions containing dispersions of metal particles such as silver metal particles to substrates, followed by curing the photocurable components in the photocurable compositions. The applied silver particles in the cured compositions can act as catalytic (seed) particles for electrolessly plated electrically-conductive metals. Useful electrically-conductive grids prepared in this manner are described for example, in U.S. Pat. No. 9,188,861 (Shukla et al.) and U.S. Pat. No. 9,207,533 (Shukla et al.) and in US Patent Application Publications 2014/0071356 (Petcavich) and 2015/0125596 (Ramakrishnan et al.). Using these methods, photocurable compositions containing catalytic silver particles can be printed and cured on a suitable transparent substrate, for example, a continuous roll of a transparent polyester film, and then electroless metal plating can be carried out on the catalytic silver particles. However, these methods require that high quantities of purchased silver particles be uniformly dispersed within the photocurable compositions so that coatings or printed patterns have a sufficiently high concentration of catalytic sites. Without effective dispersing, silver particles readily agglomerate, leading to less ineffective electroless plating and electrical conductivity.
Moreover, forming stable patterns of silver particles in this manner requires the presence of photosensitive components such as polymerizable monomers or cross-linkable polymers that must be exposed to suitable radiation. Scaling such curing procedures to high volume use can be difficult and hard to reproduce on a consistent scale, especially to produce fine line electrically-conductive meshes or grids where the uniformity and size of fine lines are subjected to highly rigorous standards.
Efforts are being directed in the industry to avoid the need for photocuring. For, example, U.S. Patent Application Publication 2012/0225126 (Geckeler et al.) describes a solid-state method for preparing silver nanoparticles using a mixture of a silver salt and a water-soluble polymer such as a starch or cellulose derivative that acts as a silver ion reducing agent. The mixture is milled by a high-speed vibration milling process to form silver nanoparticles within the water-soluble starch or cellulosic polymer so that a solvent is not needed for synthesis or transportation of the silver nanoparticles.
Various methods have been employed in the production of silver nanoparticles, such as co-precipitation methods in an aqueous solution, electrochemical methods, aerosol methods, reverse microemulsion methods, chemical liquid deposition methods, photochemical reduction methods, chemical reduction methods in a solution and UV irradiation methods. However, the conventional technologies have difficulties in the control of particle sizes and large-scale production of particles.
There are a variety of methods for producing nanometer-sized metallic nanoparticles. For example, U.S. Pat. No. 6,572,673 (Lee et al.) discloses a process for preparing metal nanoparticles, comprising reacting suitable metal salts and anionic surfactant containing an anionic group such as a carboxylic group, sulfate group, or sulfonate group as reducing agent in water under reflux at a temperature of 50-140° C. Such processes are carried out in aqueous solutions.
U.S. Pat. No. 9,005,663 (Raghuraman et al.) discloses a method for making silver nanoparticles, comprising reacting a silver salt with a phosphene amino acid. However, the phosphene amino acid reactant is an expensive material.
U.S. Pat. No. 7,892,317 (Nia) discloses a process for the synthesis of silver nano particle, consisting of reacting silver salt and an anionic surfactant, or a nonionic surfactant, and a reducing agent in an aqueous solution at room temperature.
U.S. Pat. No. 9,496,068 (Kurihara et al.) discloses a process for the synthesis of amine coated silver nano particles via thermal decomposition of oxalate ion-alkylamine-alkyl diamine-silver complex.
U.S. Patent Application Publication 2010/0040863 (Li) discloses a process for producing carboxylic acid-stabilized silver nanoparticles by heating a mixture of a silver salt long alkyl chain carboxylic acid and a tertiary amine in methanol.
U. S. Patent Application Publication 2014/0312284 (Liu et al.) discloses a process for producing an organoamine stabilized silver nanoparticle by reduction of silver salts with hydrazine in methanol. However, hydrazine is a toxic material and it would not be desirable to include it in a manufacturing process.
Cellulose is a polydisperse linear homopolymer consisting of regioselective and enantioselective β-1,4-glycosidic linked D-glucose units. The homopolymer contains three reactive hydroxyl groups at the C-2, C-3 and C-6 atoms that are in general, accessible to the typical chemical conversions of primary and secondary —OH groups.
The use of cellulose together with its derivatives has wide spread applications, for example in fibers, films, plastics, coatings, suspension agents, composites. With the advent of synthetic polymers, their uses have somewhat diminished, but cellulose derivatives are still the raw materials of choice for some uses. In addition, various studies are on-going to look for and expand their use in existing and new technologies. Cellulosic polymers can be considered renewable resources in some instances. An inherent problem that faces users of cellulosic polymers is their general insolubility in most common solvents. Modifying the structure of cellulosic polymers can improve their solubility, leading to the synthesis of various cellulose derivatives (cellulosics) that come in all forms and structures depending on the functional group(s) used in place of the hydroxyl groups on the cellulose chain.
For example, cellulose derivatization can involve partial or full esterification or etherification of the hydroxyl groups on the cellulose chain by reaction with various reagents to afford cellulose derivatives like cellulose esters and cellulose ethers. Among all cellulose derivatives, cellulose acetate is recognized as the most important organic ester of cellulose owing to its extensive industrial and commercial importance. Properties of cellulose derivatives (esters and ethers) are determined primarily by the functional group. However, they can be modified significantly by adjusting the degree of functionalization and the degree of polymerization of the polymer backbone to modify solubility in various solvents.
The solution properties of cellulose acetates have been well studied and have been shown to be influenced by the average degree of substitution and the distribution of substituents along the chain. Previous work on the gelation mechanism of cellulose acetate has shown interesting behavior with respect to the sol-gel transition. Cellulose acetate gels exhibit thermally reversible properties that depend on factors such as concentration, acetyl content, and the type of solvent. It is usually difficult to predict if cellulose will gel in a given organic solvent, and in most cellulose acetate/solvent systems, gelation occurs after the solution is heated to a specific temperature and subsequently cooled. For example, Kwon et al., Bull. Korean Chem. Soc. 26(5), 837-840 describe a study of silver nanoparticles in cellulose acetate solutions.
U.S. Ser. No. 15/456,686 (noted above) describes a method for preparing articles using silver nanoparticles that are obtained by thermal reduction of reducible silver ions in the presence of certain cellulosic polymers.
Besides silver particles, copper particles (including nanoparticles and microparticles) and gold particles (including nanoparticles and microparticles) are the most significant metallic particles that have found wide application, especially in electronics and catalysis, due to their excellent electrical conductivity, catalytic abilities, and high chemical stability. As for silver, several different methods have been devised for the preparation of copper particles having dimensions in the range of from tens of nanometers to micrometers, some of the methods being more suitable that others. However, apart from indisputable benefits, the methods that are currently used also exhibit several disadvantages.
The most widely used methods for preparation of copper particles are based on liquid-step reduction, in which metallic copper is produced by reduction from a suitable precursor dissolved in a suitable solvent by using a suitable reducing agent. The mean size of the particles thus prepared depends on the ratio of the copper precursor to the reducing agent, as well as on the specific reaction conditions (for example, temperature, stirring conditions, order of mixing individual components together, and addition of other reagents). The most commonly used copper precursors are copper sulfate, copper chloride, copper nitrate, copper acetylacetonate, copper acetate, cuprous oxide, and cupric oxide. For dissolution of these copper precursors, different solvent systems are used, most often on the basis of water, organic solvents (such as acetone or toluene), alkanes (such as n-hexane, n-heptane, and n-octane), ethylene glycol, polyethylene glycols, or various mixtures thereof. As a reducing agent especially sodium borohydrate (NaBH4) or potassium borohydrate (KBH4) is used [see, for example Lisiecki et al., Journal of the Physical Chemistry 100 (1996) 4160, or Zhang et al., Nanoscale Research Letters 4 (2009) 705] as well as hydrazine [see, for example, Wu et al., Journal of Colloidal Interface Science 273 (2004) 165], sodium hypophosphite [see, for example, Lee et al., Nanotechnology 19 (2008) 415604], and ascorbic acid in an alcohol-water mixture [see, for example, Yu et al., Nanoscale Research Letters 4 (2009) 465].
U.S. Patent Application Publication 2012/0251381 (Bedworth et al.) discloses a method for preparation of copper particles by using liquid-step reduction.
U.S. Patent Application Publication 2008/0159902 (Shim et al.) describes the preparation of copper particles having a diameter below 100 nm by a sequence of dispersion of Cu2O or CuO microparticles in a hot solution of an amine compound and subsequent chemical reduction in a mixture of oleic acid and formic acid.
WO 2009-040479 (Maijala et al.) discloses the preparation of copper nanoparticles with a diameter ranging from 1 nm to 10 nm using copper chloride and NaBH4 in an ethanol-chloroform solution.
The disadvantage of all these methods is that they require long reaction times, a great excess of the reducing agents, high temperatures and pressures and, above all, the use of exotic and often highly toxic substances, which makes these methods difficult to be scaled up and used in an industrial environment. Moreover, the final price of the produced copper particles can be negatively influenced to a considerable extent by a potential use of other additives, such as surfactants (for better wetting of the produced particles), particle stabilizers, and other necessary process additives. Another drawback is the necessity to use a highly expensive process of bubbling inert gases through a reaction mixture in order to avoid undesirable oxidation of the produced copper particles.
Therefore, there is a need to eliminate the disadvantages of the known processes for producing either silver or copper nanoparticles. Despite all the various approaches and efforts to provide electrically-conductive silver or copper in various consumer and industrial articles, there remains a need for simpler and less expensive compositions and methods for generation of silver nanoparticles in a fashion suitable particularly for pattern formation in high speed manufacturing processes at low temperature using minimally toxic organic solvents.